The purpose of this study is to develop techniques for studying central nervous system monoamine metabolism in man, and to carry out pharmacokinetic studies of psychoactive drugs. Studies of the kinetics and clearance of deuterated homovanillic acid (HVA) from the circulation were conducted in rhesus monkeys and patients. HVA distributes in the body according to a two compartment model, and is cleared with a beta half life of approximately 45 minutes. Infusion of deuterated homovanillic acid, in combination with probenecid infusion, may provide a method for estimation of the rate of production of HVA by the central nervous system in man. CSF probenecid levels measured during the oral probenecid test are significantly correlated with elevations in HVA and 5HIAA throughout the range of probenecid concentrations encountered. Studies on CSF norepinephrine (NE) levels suggest that it is a more satisfactory indicator of brain NE metabolism than NE metabolite levels in CSF.